GREENVILLE, S.C. - Planned Parenthood, the courts and Mitt Romney all came under attack on Wednesday from four GOP presidential contenders that spoke at a forum organized by Personhood USA, a national anti-abortion group.

Texas Gov. Rick Perry, former House Speaker Newt Gingrich, former Pennsylvania Senator Rick Santorum and Rep. Ron Paul, who appeared via a live satellite feed, all affirmed their commitment to the anti-abortion cause. Perry and Gingrich also took a few shots at Romney, the only candidate not in attendance, for his famously fluid views on the issue.

Organizers said the Romney campaign said he could not attend because of a scheduling conflict and that a scheduling conflict also prevented the former Massachusetts governor from attending a similar event earlier in the year in Iowa.

Perry touted his credentials as an anti-abortion governor who defunded Planned Parenthood in his state and was behind parental notification measures, as well as a state law requiring a woman to see a sonogram before being able to obtain an abortion. He said that as president, he would veto any bill that allocated funds to Planned Parenthood, even it meant striking down legislation that provided for vital government functions.

"I've been around this business long enough to understand the horse-trading that goes on but there's some principles that you don't trade on," Perry said.

On Romney's change of heart on the issue, Perry said he found it unbelievable that the former Massachusetts governor became anti-abortion in his 50s. "It is clear to most of us that this was a choice for convenience," he said.

Gingrich also took aim at Romney for the health care bill he passed in Massachusetts, which allocated state funds for abortion clinics. The bulk of his remarks, however, focused on what has been one of his favorite punching bags throughout his presidential campaign: the courts.

"The president interprets the Constitution as president," Gingrich said. "If the court makes a fundamentally wrong decision, president can in fact ignore the courts."

Santorum, the candidate best-known for his strongly conservative views on both abortion and gay marriage, called abortion "an indispensable moral issue" and called for a president who would continue to talk about it past primary season. He tied the origins of Planned Parenthood to the eugenics movement, and, as he does frequently on the stump, related the issue to his three-year-old daughter, who has a severe genetic condition.

"Every child has dignity and value," Santorum said. "I'm the blessed father of a special needs child which is the greatest gift I've been given in my life."

Santorum said he too would veto legislation funding Planned Parenthood and said he would fight the courts when he believed they erred on fundamental issues.

Paul earned the most boisterous applause of the evening, speaking of his experience as an OB-GYN and how the invention of the sonogram dramatically changed how women made decisions regarding abortion.

Paul said he is in favor of a constitutional amendment--worded to his liking--banning abortion and changing the constitution of the courts. In the meantime, Paul advocated handing power back over the states by backing a bill that would get rid of the federal jurisdiction of the courts on abortion-related matters.